Thermal batteries are the preferred back-up power sources for many weapon and defense systems. This is due to their very long shelf life (about 40 years). Thermal batteries are kept in an essentially frozen state until activated by heating. Within milliseconds of reaching operating temperature, thermal batteries produce very high pulse power outputs. Power generated by such batteries is utilized for guidance, communication, and arming of weapon and defense systems. Accordingly, thermal batteries play a critical role in our national defense.
Quality control is critical in thermal batteries in military systems, given that they are activated (for example by heating) only once to be used instantly, this after being dormant for perhaps 25 years. The reliability of these systems is gauged indirectly on the reliability of like-constructed components.
The prevailing construction and chemistry of a state-of-the-art thermal battery comprises Li-alloy and metal sulfide electrodes, with a lithium halide salt as the electrolyte. Interleaved between the electrodes and residing within the electrolyte are separators. The salt becomes molten upon heating. The separator component of a thermal battery physically separates and ionically couples the anode and the cathode in each cell of the battery. Ideally, a separator should have a relatively high capacity for an electrolyte and have connected porosity for high performance. Added characteristics of importance include dimensional stability and robustness.
Typical batteries comprise a stack of wafers of pelletized powders. Wafer fabrication and battery assembly involve substantial hand labor, partly due to the frangible nature of separators. The wafer pressing operation has received some automation, but battery assembly typically relies on hand stacking of components.
Embodiments of state of the art thermal batteries include Li-alloy (and mainly LiSi alloy) anodes, molten halides as electrolytes and a FeS2 cathode. To obtain temperature and chemical stability, they were built around MgO pressed-powder separators, therefore embodying a ceramic approach to construction. Lower melting salt (e.g. nitrates) applications are based on the ceramic approach to battery construction.
Lower temperature technologies based on Li-ion or aqueous-type of electrochemical cells, such as lead acid, typically utilize polymer separators.
U.S. Pat. No. 5,382,479 Jan. 17, 1995 is an example of the typical stacked-pellet construction for thermal battery. These ceramic based separators require pyrotechnic heating to greater than 350° C. to activate the molten salt electrolyte chemistry.
U.S. Pat. No. 6,544,691, Apr. 8, 2003, discloses lower temperature operation with nitrate molten salts among other organic salts. The claims include traditional chemistry (anode/cathode, e.g. Li-alloy/metal sulfide or metal oxide) and construction with pressed-powders including ceramic separators.
U.S. Pat. No. 4,260,667 Apr. 7, 1981 discloses an electrolyte oxidizer and U.S. Pat. No. 7,629,075 Dec. 9, 2009 discloses chloride free molten nitrates with Li-alloy electrode
Shinohara et al., U.S. Pat. No. 6,447,958, Sep. 10, 2002 discloses a battery separator comprising heat resistant nitrogen-containing aromatic polymer and a ceramic powder to include thermoplastic for shut down property.
Patent Appl. US 2007/0100012, May 3, 2007 discloses Production of high porosity membranes with polyvinylidene difluoride, PVDF. Polyolefins are the common material of choice.
US Patent Appl. US 2007/0099080, May 3, 2007 discloses molten electrolyte that contains at least one organic salt for medium temperature operation as an alternative to the lower melting-point nitrate for thermal batteries.
U.S. Pat. No. 7,871,447, Jan. 18, 2011, discloses an automated production of the stacked-pellet thermal battery that is extraordinarily controlled due to fragility of the pressed pellets and to insure reliability.
U.S. Pat. No. 7,807,286, Oct. 5, 2010 discloses production of high porosity membrane of nonwoven polyolefin fiber with a multiplicity of apertures overlaid with ceramic coating of Al, Zr and Si oxides. This membrane is limited to 120° C. operation temperatures with dependence on polymer porosity and with a shutdown feature when temperatures exceed 120° C.
Standard thermal batteries rely on pressed-powder MgO separators which are relatively inexpensive chemically stable, and which can immobilize 65-85 volume percent of electrolyte within the pores of the pressed powder. A significant drawback of MgO separators is the limited structural stability of the material. This limited structural stability relegates MgO separators to relatively thick configurations, of at least about 0.3 to 1.0 millimeters. Any thinner ceramic-based separators are impractical due to the physical strength limitations inherent in MgO pressed-powder materials. A typical MgO powder separator with molten electrolyte is transformed to a paste at thermal cell operating temperatures. These paste separator configurations limit cell structures of thermal batteries to a simple stack of pellets. By comparison, the invented thermal battery enables the construction of a wound assembly design in addition to a stacked pellet assembly, compressed in a planar configuration.
Interest in alternative thermal batteries and separator technologies is ongoing. The trend has been toward development of higher energy and power density. The design approach for this has typically involved producing thinner cells. Thermal batteries are produced from stacked cells consisting of pressed powder wafers comprising layers which are situated in the following repeating order: (a) a heat pellet, (b) a Li negative electrode (anode), (c) a porous separator (e.g., MgO) containing a molten electrolyte salt, and (d) a FeS2 positive electrode (cathode). Each wafer typically is about 0.5 mm (about 20 mils) thick. Battery energy and power density could be improved by using a thinner separator, if suitable materials were available. Improvements tend toward thinner components but generally remain planar, i.e. poker-chip pellets that are stacked to form the battery. This includes not only the electrodes and separators but also the heat pellets (pyrotechnics) that are interleafed to heat, or thermally initiate the battery stack. These thermals are based on molten alkali halide electrolyte and therefore require 400-550° C. operation.
A thinner separator would boost the proportion of active materials (electrolyte) in the battery, and thus boost power output. Unfortunately, MgO powder wafers have limited handling strength, and generally must be at least about 1 mm in thickness for practical use in thermal batteries. Thinner MgO tends to crack or break, thus compromising the integrity of the entire battery. Larger diameter wafers exacerbate the handling problems. Because of this, MgO powder wafers must have a substantial thickness to be of practical use. In addition, volumetric changes of the active electrolyte material tend to distort the electrolyte/separator interface, which leads to cell shorting.
Current thermal battery manufacturing employs uniaxial powder pressing technology to form active cell components. The thickness, diameter, and overall geometry (parts are typically cylindrical) of the wafers are limited by the uniaxial powder pressing process. The thickness obtainable for uniaxially pressed wafers for thermal batteries ranges from approximately 1 mm to about 10 mm. Production of thinner or thicker parts is notably more difficult, and commonly results in low yields, and therefore, higher costs. Thinner wafers require precise, even die loading, while thicker wafers require the use of organic binders to distribute the applied pressure evenly. Similarly, large diameter wafers are difficult to uniaxially press due to increasingly larger processing equipment required to provide the necessary mechanical loads to form the wafers—typically greater than about 10,000 pounds-per-square inch (psi). These limitations preclude many advanced battery designs.
For electrode pellet manufacture, a high tonnage press typically is required to achieve 50 volume percent active electrode materials loading. A portion of the electrolyte salt generally is combined with the electrode material to aid in the formation of suitable cold-pressed pellets. The metal sulfide electrode material, FeS2, is a very hard material and does not compact well on its own. Typically, the pressed electrode uses FeS2 coated with electrolyte salt to facilitate the powder compaction. The resulting cold-pressed pellet generally comprises about 50 volume percent FeS2, about 30 volume percent electrolyte salts, and a void volume of about 20 volume percent. An unpressed powder layer would typically have a void volume of about 50 volume percent. To achieve the desired 50 volume percent active material loading, the high tonnage press must displace about 30 percent of the void volume so as to accommodate the electrolyte salt. This is crucial, in that unpressed electrodes with a 20-30 volume percent loading of electrolyte exhibit poor performance (e.g., low energy density and low power output).
The separator material used in conventional molten salt thermal batteries is pressed from a high-surface area MgO powder. MAGLITE® S or MAGLITE® D (Calgon), and more recently MARINCO® OL (Marine Magnesium Company) magnesium oxide mixed with electrolyte salt, have been the materials of choice for pressed powder separators. Due to limited availability, alternative materials have been investigated, but only the pressed-powder MgO/salt separator has found commercial application. The separator performance is closely related to the surface properties of the MgO particles. Thermal battery production remains costly with fragile materials, expensive facilities, and extreme quality control because they can't be tested, shelved and later activated.